1. Field of the Invention
The present invention relates to standard (reference) materials for calibration of a photoelectric colorimeter (color and color-difference meter) which is used for instrumental measurement for ASTM color, said measurement being colorimetry of petroleum products such as various lubricating oils. petrolatum and microcrystalline wax. The present invention also relates to a method for Instrumental measurement for ASTM color of a petroleum product using said standard materials (standard samples).
2. Prior Art
There are two testing methods for colorimetry of petroleum products, one being a method for measuring ASTM color of petroleum products and the other a method for measuring Saybolt color thereof as prescribed in JIS K 2580.
Of these, the method for measuring ASTM color is applied to various lubricating oils and petrolatum, etc. and will indicate the color of a test material in terms of a symbol or a value ranging from 0.5 (light) to 8.0 (dark) obtained by comparing the color of the test material with that of a standard (reference) colored grass (glass color standard) by generally using an ASTM chromometer.
It is noted that the ASTM color of a material is expressed as a symbol or a value (any one of 0.5 to 8.0 in accordance with JIS K 2580) which is assigned to a standard colored glass whose color corresponds to the color of the material.
The standard colored glass used in the above method is prescribed to have its color dispersion limited as indicated in the XYZ colorimetric system in accordance with JIS Z 8722, the luminous transmittances using the CIE standard Illuminant C and the chromaticity coordinates based on the RGB color system.
However, in cases where the ASTM color of a material is measured by the use of the ASTM chromometer, the ASTM color so found is Judged by human vision and the results expressed by said measurement are not quantitative {when the thus found color of a test material used is between those (for example, 4.0 and 4.5 in ASTM color) of two different standard colored glasses, it is expressed as L 4.5 (this 4.5 is a symbol for the standard colored glass having a darker color), while when the color of the test material is darker than 8.0 in ASTM color then It is expressed as D 8.0} and, therefore, personal differences in judgment are apt to be made and time measurers practically worry about the judgement. This tendency is particularly remarkable when the color of the test material is dense or dark.
Accordingly, even in such a colorimetry as above, automatization of judgment of color without resort to visual Judgment is eagerly desired at the present time.
In view of the above, the present inventors attempted to investigate whether or not a commercially available photoelectric colorimeter (measuring instrument) can be used for measuring a test material for its ASTM color.
A photoelectric colorimeter used must be one which will produce a favorable correlation with the testing method as prescribed in JIS K 2580 in the entire region of ASTM color ranging from 0.5 to 8.0. Furthermore, there is desired a universal method for testing a material, especially a liquid, for its color.
For the perception of color with human eyes under natural light, the quantities of stimuli due to three primary colors of lights which are red, green and blue lights, are important. In measuring a color with an optical instrument, a light emitted from a light source is used instead of the natural light and what is important is not the quantities of stimuli but tristimulus values.
Accordingly, it is desirable that a method testing a test material for ASTM color be a universal method, whose performance is close to the perception of color with human eyes, based on the tristimulus values by an optical instrument and that the expression of the results of measurement be a quantitative expression without the use of L OF D color expression. Therefore, it is desirable to adopt a testing method based on methods for measuring a material (methods for measurement of reflecting or transmitting objects for their color) which are recommended by Commission Internatlonale de l'Eclairage (hereinafter abbreviated as "CIE") and prescribed in JIS Z 8722 and also to adopt a method for quantitatively expressing measured results based on the color expression method using the XYZ colorimetric system prescribed in JIS Z 8701.
In measuring a sample for its color by a photoelectric colorimeter, it is necessary to express the measured value of the sample obtained by a commercially available photoelectric colorimeter in terms of the ASTM color. Therefore, it is necessary to find the correlation between the tristimulus value and ASTM color.
Comparing the sum (.SIGMA.D) of optical densities based on the XYZ colorimetric system of standard colored glasses with the ASTM color of the same, the present inventors have found a correlation therebetween as represented by the following correlation formula: EQU A=.alpha..SIGMA.D+.beta.
wherein A is ASTM color, .SIGMA.D is the sum of optical densities (DX+DY+DZ), and .alpha. and .beta. are each a constant for calibration of instrumental errors.
Therefore, the measured value of the ASTM color is obtained by processing the measured X, Y and Z values in the operation-display section of the measuring instrument on the basis of said correlation formula.
As the above .SIGMA.D value varies with an instrumental error depending on an instrument used, it cannot always be constant even for the same material. It is necessary, therefore, to prepare at least two kinds of standard liquid materials for finding at least two ASTM colors to calibrate each instrument (photoelectric colorimeter) with the standard liquid materials.
Such standard materials are required to be low volatile, highly resistant to oxidation, low degradable, for example low oxidizable, with the elapse of time and satisfactory in color stability.